somehow it still feels like a bad idea to give all of the money I can spare exclusively to SIAI.
If you were Bill Gates, that might be a valid concern. (The "exclusively" part, not the "SIAI" part.)
Otherwise, it's most efficient to donate to just one cause. Especially if you itemize deductions.
It has been claimed on this site that the fundamental question of rationality is "What do you believe, and why do you believe it?".
A good question it is, but I claim there is another of equal importance. I ask you, Less Wrong...
What are you doing?
And why are you doing it?